To get from an endocrine cell to a target cell, a hormone must travel in the blood, which is mostly water. Most of the monoamines and peptides are hydrophilic, so mixing with the blood plasma presents no problem for them. Steroids and thyroid hormone, however, are hydrophobic and must bind to hydrophilic transport proteins to get to their destination. The transport proteins are albumins and globulins synthesized by the liver. A hormone attached to a transport protein is called bound hormone, and one that is not attached is an unbound (free) hormone. Only the unbound hormone can leave a blood capillary and get to a target cell (fig. 17.17).
Transport proteins not only enable hydrophobic hormones to travel in the blood, they also prolong their half-lives. They protect circulating hormones from being broken down by enzymes in the blood plasma and liver and from being filtered out of the blood by the kidneys. Free hormone may be broken down or removed from the blood in a few minutes, whereas bound hormone may circulate for hours to weeks.
Thyroid hormone binds to three transport proteins in the blood plasma: albumin, an albumin-like protein called thyretin, and an a-globulin named thyroxine-binding globulin (TBG). TBG binds the greatest amount. About 99.8% of T3 and 99.98% of T4 are protein-bound. Bound TH serves as a long-lasting blood reservoir, so even if the thyroid is surgically removed (as for cancer surgery), no signs of TH deficiency appear for about 2 weeks. Steroid hormones bind to globulins such as transcortin, the transport protein for cortisol. Aldosterone is unusual. It has no specific transport protein, but binds weakly to albumin and others. However, 85% of it remains unbound, and correspondingly, it has a half-life of only 20 minutes.
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.